1. Field of the Invention
This invention is directed to spectrometers that fosters and detects absorptions in submillimeter wave region, and to methods for processing spectroscopic data with unique algorithm.
2. Description of the Background
Spectroscopy using submillimeter wave, or microwave, has the potential to be a potent diagnostic tool. Molecules having a permanent or induced dipole moment have discrete rotational energy levels that can absorb electromagnetic waves in MHz to THz regions. Because of the abundance of rotational energy levels that are unique to the structure of the molecule detected, submillimeter spectroscopy makes it possible to identify chemical species with excellent specificity. One of the earliest studies in this filed was reported almost 50 years ago; “The use of microwave spectroscopy for chemical analysis,” C. H. Townes and A. L. Schawlow, chapter 18, “Microwave Spectroscopy,” McGraw-Hill, 1995. However, this analytical method did not result in a wide spread use in research communities and industries, partially because the complexity of the instrumentation and associated high cost and maintenance issues.
Recently, with the advancement of THz technologies and computer based system control, submillimeter spectrometers with more practical use have been developed. Such spectrometers are reported, for example, in “A Fast Scan Submillimeter Spectroscopic Technique,” D. T. Petkie, T. M. Goyette, R. P. A. Bettens, S. P. Belov, S. Albert, P. Helminger, and F. C. De Lucia, Rev. Scient. Instrum 68, 1675-1683 (1997), and “FASSST: A new Gas-Phase Analytical Tool,” S. Albert, D. T. Petkie, R. P. A. Bettens, S. P. Belov, and F. C. De Lucia, Anal. Chem. 70, 719A-727A (1998).
This type of spectroscopic technique was initially made possible by a combination of fast scanning voltage tunable Backward Wave Oscillators (BWOs), optical calibration methods, and modern fast digitization and computation techniques. FIG. 1 shows a diagram of such a spectrometer. Briefly, the BWOs are voltage tunable (˜1500-4000 V) tubes covering roughly 0.1-1.0 THz, in bands. A typical tube, an OB-30, covers ˜250-350 GHz as the voltage is swept over this range. In this system the output of the BWO is split, with about 10% going to a Fabry-Perot cavity to provide optical calibration, while the rest is used to interrogate the sample. The portion of the power then passes through the sample cell and is detected by an InSb detector. In operation, the tuning voltage is ramped to provide an analog sweep in frequency, and the outputs of the signal detector and FP detector are recorded in parallel.
A typical sweep time is one second and the frequency width 10-100 GHz. This frequency interval contains ˜105 independent resolution elements. With integration times of 1 microsecond, S/N ratios of 104 are obtained.
The system concept is based on the short term spectral purity of the BWO (Q>107). Because of this spectral purity, the slow and complex phase lock that is ordinarily used to stabilize and control THz spectroscopic systems is not fundamentally necessary. In this spectrometer, the high spectral purity source is swept so rapidly that slow instabilities (associated with thermal drift, etc) are frozen on the time scale of a measurement cycle. Fast digitization records the output of the FP cavity and spectrometer in parallel and makes possible accurate frequency calibration even with thermal drifts, power supply ripple, and nonlinear frequency sweeps. In some sense, the speed of the digitization plays the same role as the bandwidth of a more traditional phase lock loop.
The scanning speed combined with the high spectral brightness (W/Hz) of electronic sources and the very strong molecular interactions in the THz range, rapidly produces analytical fingerprints with remarkable information content. This is illustrated in FIG. 2, which is presented as a series of blow-ups in both frequency and amplitude.
FIG. 2 shows about a 25 GHz region of the spectrum of HNO3, which was recorded in 1 second. The middle and lower figures show expansions of this spectrum.
Stated another way, if the upper figure were plotted with 1 mm of noise in the vertical and 1 cm of width per resolution element in the horizontal, it would require a piece of paper 10 m high and 1 km long for each second of data acquisition.
The high resolution of the millimeter spectroscopy comes about because the Doppler broadening (which is the fundamental limit in most systems) is proportional to frequency. Thus, lines in THz spectra are about 100 times narrower than those in infrared spectra. For example, for molecules of about the size of ClONO2, their spectra are unresolvable in the infrared. Rather than the detailed rotational spectra shown in FIG. 3, only a broad band is observable in a infrared spectrum. Because of the high resolution in the THz, the optimum pressure (set by the condition that the Doppler and pressure broadening be approximately equal) is much lower, typically 10-50 mTorr.
One of the disadvantages of this spectrometer is, however, its size. The Fabry-Perot cavity alone occupies a space as large as a small room. In addition, BWO and associated magnets for guiding electrons as well as the InSb electron bolometer that needs to operate at 1.5 K require plenty of space for its instrumentation. Because of its excellent specificity and speed, the submillimeter spectrometer is theoretically suited for applications outside laboratories such as remote chemical analysis. However, at least the large size has prevented the submillimeter spectrometer from being seriously considered for such applications.